What did they say ? Have you contacted them ? Mine has similar issue, especially visible at DCI 4K mode - i would say it look like as some kind of vertical stripes in dark areas (underexposed environment), they are visible on j-log footage at iso starting from 960-1250, interesting fact that 1600 is better than iso 1250 for example (just grain noise a bit). At ISO below 800 they are very slight. At 560 and down to 400 image is absolutely clean. I Because i filming mostly in J-log - i try not to go above ISO 800. At UHD situation is better. It is image sensor related issue - i can see all these issues very clearly on external 4K monitor via HDMI.

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